1. Field of the Invention
The invention relates generally to surface acoustic wave devices such as couplers, resonators, or filters in which it is desired to obtain a wide bandwidth device simultaneously having low losses and preferably with very low sidelobes and high ultimate rejection.
2. Description of the Prior Art
A surface acoustic wave device having a wide bandwidth, low insertion loss, and low sidelobes or steep passband skirts has long been sought for applications such as IF filtering or radar signal processing because of its relatively small size and ease of fabrication compared with lumped constant devices. In most surface wave devices an input transducer launches surface waves in two opposed directions upon excitation by an externally applied electrical current. However, less than half of the energy within the surface waves produced by the input transducer ever reached the output transducer or surface wave energy extraction means in most previously known devices as the output transducer lay along a linear surface wave propagation path from the input transducer and only received surface waves launched from one of the two directions. In order to overcome these losses, devices were constructed using a non-planar substrate having a surface in the form of a circle or ellipse or two parallel planar surfaces with circular portions. Surface waves launched by the input transducer traveled around the device propagating in both directions around the circular portions of the device to the output transducer. Unfortunately, such devices did not achieve as low an insertion loss as was desired for many applications because a substantial portion of the surface waves were converted to bulk waves as they propagated over non-planar portions of the substrate. The proportion of the surface wave energy converted to bulk waves was lost prior to reaching the output transducer.
Another technique that has been utilized to minimize the bidirectional energy loss has involved the use of complex interdigital transducer designs along with various techniques for adjusting the relative phase of portions of the transducer. All of these so-called unidirectional transducer structures suffer from one or a number of deficiencies. These deficiencies include: narrow passband, poor sidelobe suppression, poor ultimate rejection, difficult fabrication, and poor shape factor.
The attainment of wide bandwidth with low inband loss and high ultimate rejection (the signal rejection outside of the passband of the device) has also been long sought. In general, in previously known wideband devices, the passband of the device was determined for the most part by the passbands of the input and output transducers and the interactions therebetween. Devices have been described in which the overlap of the conductive fingers within the input and output transducers is varied in accordance with predetermined frequency characteristics. Although these techniques have met with some degree of success in improved bandwidth, devices were not constructed which simultaneously have sufficiently wide bandwidth, low inband insertion loss and high ultimate rejection to be useful in many radar signal processing filtering applications.
Resonator-type devices are known in which input surface waves launched by the input transducer propagate around a closed path many times past the output transducer setting up a standing wave pattern along the entire propagation path. By this technique, very narrow bandwidths, typically no more than 0.1% have been obtained. However, low sidelobes and steep skirts have not been demonstrated.